Problema com linhas de fluxo no ArcMap

GeorgiosAlevizos

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Olá comunidade, aqui é o George!<\/P>

Como parte do meu PhD, preciso mapear os rios do Norte do Peloponeso, Grécia. Baixei dois DEMs do Nasa Reverb, que conectei com "mosaic to new raster". Comecei o processo usual de "fill", "flow direction" e "flow accumulation" para poder analisar os rios da área. Mas infelizmente estou enfrentando dois problemas.<\/P>

O primeiro é que os resultados que obtenho da ferramenta Flow Direction estão errados e não consigo encontrar a solução, não importa o que eu tente. Como você pode ver na primeira captura de tela, há algumas linhas de fluxo horizontais que não fazem sentido. Tentei editar os valores (tentei fazer o arcmap excluir valores menores que 1) do raster original e depois usar "fill", mas isso não impede a ferramenta "fill" de ignorar essas áreas (terceira captura de tela).<\/P>

O segundo problema (que parece estar conectado ao primeiro) é que decidi avançar para a próxima etapa e usar "Flow Accumulation" mesmo assim, pois algumas áreas pareciam estar corretas e ver o que acontecia.<\/P>

Como você pode ver na segunda captura de tela, as linhas de acumulação de fluxo seguem a forma da linha costeira e depois se estendem para o mar. Pelo que entendo, essas linhas de acumulação de fluxo seguem as linhas de direção do fluxo, então o problema é criado quando uso a ferramenta "flow direction".<\/P>

Você poderia me ajudar? Sou relativamente novo nisso então desculpe minha ignorância.<\/P>

Se for um problema com os dados originais e não puder ser corrigido, existe alguma alternativa?<\/P>

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Obrigado!<\/P>

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MarkBoucher

Georgios,

I found the area you are working in using Google Maps (see attached image). The area where you have the strange lines is in the water (Gulf of Corinth). If the elevation in your DEM is level in the water area, the flow direction would be somewhat random. Since you are dealing with a coastal area, you will have some unique issues to deal with. However, I don't see anything wrong with the results. There was some recent discussion on coastal drainages. Here is the link to that includes that discussion: Re: Catchment vs watershed.​

You are right that the flow accumulation is based on the flow direction. If the flow direction is wrong (has errors) the rest of the analysis will have errors.

Since you have creeks, rivers, and streams that have to cross under roads on their way to the sea, I suspect you will want to create an agreestream layer and "burn" the streams into the rawdem at the road crossings so the flow direction will be correct at those locations. It looks like the topography is steep enough that for the most part you won't need to burn in streams anywhere else except in very flat place or where man-made drainage systems control the flow direction instead of the topography.

So, basically you are not doing anything wrong to get the "horizontal flow lines". A possible way to get rid of them would be to use the Con() function to make every raster grid below a certain elevation (i.e. in the water) "Null". I'd have to do a bit or research myself to give you direction on how to do that. I'm sure there is a tool in Spatial Analyst that would do work. This link might show you how. ArcGIS Desktop Help 9.3 - Setting values to NoData: SetNull.​ Once the raster has null (or "no data") values in the water area, it is possible that the flow direction grid will not be defined there and so you won't get flow accumulation there either.

The flow direction grid is only a foundational layer and will not show in your final analysis where I assume you will define watersheds. You might simply move on and establish your analysis process for a small area until you settle on a process you are happy with. Then perform that process on the whole dataset.  I use Model Builder in this way. I allows me to build and edit the process and they reapply it on different datasets.

Best,

Mark